Rice Genetics IV - IRRI books - International Rice Research Institute

extend to the S phase. Furthermore, treatment of seedlings with hydroxyurea, whichblocks cells in the early S phase, inhibited the patchy expression of cdc2Os3 at theroot apex, whereas transcripts were still detectable in roots treated with colchicine,which blocks cells in mitosis. The patchy pattern on colchicine-treated sections mightreflect the partial synchrony of cell division in this case. Thus, transcripts of cdc2Os3appeared to be abundant from the G2 to the M phase but almost disappeared whencells had completed mitosis at telophase.Both Cdc2Os1 and Cdc2Os2 include the characteristic PSTAIRE domain(Hashimoto et al 1992) and are classified as PSTAIRE CDKs on the phylogenetic tree.PSTAIRE CDKs, such as products of cdc2aAt in Arabidopsis, Amcdc2a and Amcdc2bin Antirrhinum, and cdc2MsA and cdc2MsB in alfalfa, are expressed throughout the cellcycle (Magyar et al 1997). Thus, the products of cdc2Os1 and cdc2Os2 can also beclassified as PSTAIRE CDKs in terms of their pattern of expression. Several plant CDKswith altered PSTAIRE motifs have been reported, namely, the products of Amcdc2cand Amcdc2d in Antirrhinum; cdc2MsC, cdc2MsD, cdc2MsE, and cdc2MsF in alfalfa;and cdc2bAt in Arabidopsis. Rice cdc2Os3 encodes a PPTALRE sequence, which is thesame as those of AmCdc2c, Cdc2MsD, and Cdc2bAt. However, when Cdc2Os3 wascompared with the other non-PSTAIRE CDKs in the whole region, it is located close toAmcdc2d and cdc2MsF rather than to AmCdc2c, Cdc2MsD, or Cdc2bAt. Transcriptsof cdc2s belonging to the group including cdc2Os3 are abundant from the G2 to the Mphase (Magyar et al 1997). On the other hand, Amcdc2c is expressed from the mid-Sphase to early M phase, and cdc2bAt is preferentially expressed during the S and G2phases. Transcripts of cdc2MsD are abundant at the G2/M phase and are also detectedjust after alfalfa cells are released from arrest by aphidicolin (Magyar et al 1997). Therefore,we propose that the products of cdc2Os3, Amcdc2d, and cdc2MsF form a distinctsubclass of non-PSTAIRE CDKs that are preferentially expressed from the G2 to the Mphase. Our results also indicated that domains other than the PSTAIRE region are importantfor the function of non-PSTAIRE CDKs in distinct cell cycle phases. The alfalfagenes cdc2MsC and cdc2MsE as well as rice R2, which also encode divergent PSTAIREsequences, are expressed throughout the cell cycle, and the encoded amino acid sequencesare distinct from those of other CDKs (Magyar et al 1997).The level of Cdc2Os3 protein was reduced by treatment of cultured cells with hydroxyureabut not with colchicine. The histone H1-kinase activity associated withCdc2Os3 was correlated with the level of the protein. Expression of cdc2Os3 is probablycontrolled at the transcriptional level and Cdc2Os3 protein probably accumulatesfrom the G2 to the M phase.When rice R2 was overexpressed in a CAK-deficient mutant of budding yeast, itsuppressed the temperature sensitivity of the mutation (Yamaguchi et al 1998). Immunoprecipitatesof rice proteins with the anti-R2 antibody phosphorylated human CDK2,one of the rice CDKs (Cdc2Os1), and the carboxy-terminal domain (CTD) of the largestsubunit of RNA polymerase II of Arabidopsis. Mutational analysis indicated that R2phosphorylated the threonine residue within the T loop of CDK2 and Cdc2Os1. R2 wasfound mainly in two protein complexes that had molecular masses of 190 kDa and 70kDa, whereas the CDK- and CTD-kinase activities associated with R2 were identifiedMolecular dissection of cell death in rice 371